Sculpting not working
Select the grab tool (G) and ensure the brush is large enough.
To sculpt properly, you need enough polygons to operate on.
If you don't have them, consider retopologizing via the remesher option in the sculpting mode and experiment with different voxel-size levels (smaller makes more polygons) of to find the outcome you want!
Don't go too low too quickly or you will lose the ability to make coarse changes to the sculpt: One's workflow should begin coarse and become fine over time when sculpting in detail-passes!